1. Field of the Invention
The present invention relates to a contact type document reading device which is built in a facsimile equipment or the like to read a document, and more specifically, to an improvement in the wiring arrangement of conductors used to electrically connect a multiplicity of light receiving elements and driving elements for driving the light receiving elements.
2. Description of the Prior Art
FIG. 1 shows an equivalent circuit of a general document reading device of the type referred to above, and FIG. 2 schematically shows the structure of a light receiving section 100 in the device of FIG. 1. More particularly, as shown by a fragmentary plan view in FIG. 2(a) and by a sectional view in FIG. 2(b), the light receiving section 100 comprises divided electrodes 1n1 (111, 121, 131, 141, . . . ) of thin films made of an electrically conductive material such as Al, Cr or Au; a photoconductive thin film or layer 102 made of an amorphous semiconductor such as Se-As-Te or a-Si or of a polycrystalline semiconductor such as CdS or CdSe; and a transparent film or electrode 103 made of an electrically conductive transparent material such as SnO.sub.2 or ITO; the divided electrodes 1n1, photoconductive layer 102 and transparent electrode 103 being laminated sequentially on an insulating substrate BD made of glass or ceramic. That is, the photoconductive layer 102 is sandwiched by two types of electrodes, i.e., the divided electrodes 1n1 and transparent electrode 103. The light receiving section 100 is represented by an equivalent parallel circuit of a photodiode PD and a capacitor C as shown in FIG. 1. Consider a light receiving element 110, for example. The capacitor C of the element 110 is a sum of the capacity of the element 110 itself and the capacity which a conductor 210 has. These light receiving elements 110, 120, . . . 1n0 are arranged by a predetermined number which provide a density (for example, 8 dots/mm) necessary for desirable document resolution, in its main scanning direction.
The operation of the document reading device will next be explained briefly by referring to FIG. 1. In the drawing, when a shift register 500 is driven first time and MOS FETs 310, 320, . . . 3n0 are sequentially turned ON and OFF, closed loops will be accordingly established sequentially between a power source PS and the light receiving elements 110, 120, . . . 1n0, so that a predetermined amount of charge will be stored in the capacitor C. In a document reading mode, the charges stored in these capacitors C will be discharged or held therein depending on the amount of lights received at the associated photodiodes PD. Thereafter when the shift register 500 is driven second time to sequentially turn ON and OFF the MOS FETs 310, 320, . . . 3n0 to again charge the capacitors C, a current corresponding to the amount of charge stored in the capacitor C of each light receiving element will flow through a signal line 600. This current is sent from an output terminal OUT as a read signal of the document reading device. Such operation will be repeatedly executed for each scanning of the reading document. Such a document reading device as described above as generally called charge storage type document reading device.
In the document reading device, the light receiving section 100 is constructed by an array of the divided electrodes 1n1, photoconductive layer 102 and transparent electrode 103 sequentially provided on the same insulating substrate BD by means of an evaporation process, a sputtering process, a CVD or the like processes. Since the MOS FETs 310, 320, . . . 3n0 and the shift register 500 are separately formed, means for electrically connecting at least between the light receiving element 110, 120, . . . 1n0 and MOS FETs 310, 320, . . . 3n0 must be provided on the substrate BD or another substrate by wire bonding or like bonding means. That is wiring conductors 210, 220, . . . 2n0 for connection between the light receiving elements 110, 120, . . . 1n0 and the MOS FETs 310, 320, . . . 3n0 must be provided therebetween by the aforementioned wire bonding or like bonding means. In practice, the wired conductors 210, 220, . . . 2n0 become considerably long.
In view of such circumstances, in the prior art contact type document reading device, efforts were made that the conductors connecting between the light receiving elements and drive elements such as MOS FETs are made as short as possible and yet have sufficient allowable width and conductor-to-conductor (line-to-line) spacing. However, as the number of light receiving elements for the drive elements is increased and therefore the light receiving element array becomes longer, variation of the line-to-line capacities among the conductors become great, and, undesirable effect on the document reading operation will occur through the light receiving elements.
FIG. 3 shows a circuit diagram for explaining the principle of the document reading device corresponding to one bit, in which elements having substantially the same functions as those in FIG. 1 are denoted by the same reference numerals or symbols. In FIG. 3, the capacitance of the storage capacitor C usually consists dominantly of the line-to-line capacity of a wiring conductor section 200. For this reason, when the line-to-line capacity of the conductor section 200 greatly varies from bit to bit, the light receiving elements will produce a read signal having an irregular level. This results in that, even if a completely white (or black) line is scanned, the document reading device will produce an unstable read signal as shown, for example, in FIG. 4. As a result, in the prior art document reading device, it has been indispensable to provide a proper level correcting circuit to correct the irregular output signal.